Effect of heat treatment on strain-controlled fatigue behavior of cast Mg-Nd-Zn-Zr alloy

doi:10.1016/j.jmst.2018.05.001

Abstract

Abstract:

The influence of heat treatment on the strain-controlled fatigue behavior of cast NZ30 K alloy was investigated. Compared with the as-cast and solutionized (T4) alloys, the peak-aged (T6) and over-aged (T7) counterparts have a higher cyclic stress and a lower plastic strain value due to the precipitation strengthening. The as-cast and T4-treated alloys have a higher fatigue strength/yield strength ratio than the aged alloys, which is mainly attributed to their higher cyclic hardening. Under stress-controlled loading, the aged alloys show lower hysteresis energies than the as-cast and T4-treated counterparts, leading to longer fatigue lifetimes. For the T4-treated alloy, the cyclic hardening and fatigue failure are controlled by the dislocations-slip and twinning, while for both the as-cast and T6-treated counterparts, they are controlled by the dislocation-slip. For the T7-treated alloy, cyclic deformation and failure behavior are mainly dependent on dislocations-slip and grain boundary sliding.

Key words: Magnesium alloys, Heat treatment, Strain-controlled fatigue characteristics, Hysteresis energies, Cyclic deformation

Zhenming Li, Qigui Wang, Alan A. Luo, Jichun Dai, Hui Zou, Liming Peng. Effect of heat treatment on strain-controlled fatigue behavior of cast Mg-Nd-Zn-Zr alloy[J]. J. Mater. Sci. Technol., 2018, 34(11): 2091-2099.

Figures/Tables 10

Table 1 Tensile and compressive yield strengths and fatigue strengths of the as-cast and heat-treated NZ30 K alloys.

State	Fatigue strength (MPa)	Tensile testing			Compressive testing
		Elongation	YS (MPa)	FS/YS	YS (MPa)	FS/YS
As-cast	73	9.5%	92	0.79	96	0.76
T4	72	15.6%	88	0.82	89	0.81
T6	90	6.9%	165	0.55	168	0.54
T7	83	12.7%	138	0.60	139	0.60

Fig. 1. Tensile and compressive true stress-logarithmic plastic strain curves of the NZ30 K alloys under different states.

Fig. 2. Cyclic stress response of the NZ30 K alloys under different states: (a) as-cast and T4; (b) T6 and T7.

Fig. 3. SEM micrographs showing the cyclic deformation behavior the of the NZ30 K alloys under different states: (a) as-cast; (b) T4; (c) T6; and (d) T7.

Fig. 4. Mean stress of the NZ30 K alloys under different states: (a) as-cast and T4; (b) T6 and T7.

Fig. 5. (a) Average modulus of the as-cast and T4-treated alloys, (b) average modulus of the T6- and T7-treated alloys, (c) plastic strain amplitude of the as-cast and T4-treated alloys, and (d) plastic strain amplitude of the T6- and T7-treated alloys.

Fig. 6. (a) Hysteresis loops of the as-cast and T4-treated alloys, (b) hysteresis loops of the T6- and T7-treated alloys, (c) plastic strain energy density of the alloys under different states, (d) total strain amplitude energy density of the alloys under different states. Life prediction results using (e) plastic strain energy density and (f) total strain energy density.

Fig. 7. (a) Strain-life curves, (b) variation in the maximum stress amplitude as a function of cyclic numbers for the alloys. (c) Coffin-Manson and Basquin plots, and (d) plots of Δσ/2 vs plastic strain amplitude Δεp/2 for the alloys under different states.

Table 2 Strain-controlled fatigue parameters of the NZ30 K alloys under different states.

State	ń	?(MPa)	b	ε_f? (%)	c	σ_f? (MPa)
As-cast	0.27	639	-0.13	6.81	-0.39	397
T4	0.23	493	-0.12	6.5	-0.41	361
T6	0.31	1125	-0.1	7.06	-0.43	364
T7	0.23	591	-0.11	2.1	-0.3	362

Fig. 8. (a) Stress-life curves of the as-cast and heat-treated alloys. (b) Variation of stress amplitude with the number of cycles for the alloys tested at the strain amplitude of 0.2%. (c) Weibull plots showing the influence of heat treatment on fatigue life of the alloys. (d) Stress-plastic strain hysteresis loops at maximum stress values of 90-94 MPa and different total strain amplitudes (as-cast: 0.3%, T4: 0.26%, T6: 0.2% and T7: 0.27%). (e) Relationship between fatigue lifetime and size of the grain acting as initiation sites, and (f) variation of stress amplitude-fatigue life curve of the alloys together with the prediction of MSF life models.

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